1. Field of the Invention
The present invention relates to a method for improving the selection capability of a TSB (Transcoder and Selector Bank) in a mobile communication system, and more particularly for performing effective selection in the presence of delay between frames resulting from a soft handoff operation.
2. Description of the Prior Art
A mobile communication system, such as PCS (Personal Communications Services) and CDMA (Code Division Multiple Access), is generally composed of a large number of base stations (BS) which include several base station transceiver subsystems (BTS) operating under the control of a base station controller (BSC). These systems further includes base station managers (BSM) managing many BSCs, mobile switching centers (MSC) and location registration systems for tracking the position of mobile stations (MS) within the system. Each base station has an associated coverage area generally referred to as a cell. A cell is generally divided into three sectors. A communication system taking a cell as the basic unit of coverage area is called a cellular system. As an MS moves to the periphery of one cell towards another, an ongoing call of the MS will be automatically transferred from one BS to another in a process known as handover or handoff.
Normally, an MS communicates with a BTS serving the sector of the cell in which the MS currently resides. A channel operating from a BS toward an MS is referred to as a forward channel and a channel formed from MS toward BS is referred to as a reverse channel.
The forward channel includes a pilot channel, a sync channel, a paging channel and several forward traffic channels. The reverse channel includes an access channel and a reverse traffic channel. The MS and BTS communicate voice and data information over one of the traffic channels. The received signal strength of the pilot channel is used to determine when handoffs will occur.
Each BTS is assigned several operating frequencies on which a number of channels are established according to system capacity. We call each frequency channel a FA (Frequency Assignment). A CDMA system can include a great number of access channels in accordance with differing requirements from frequency offset and sequences per frequency channel.
As an MS operates near the coverage border of a cell, the wireless telephone system effects a handoff between a currently operating base station and one or more additional base stations. It is an objective of the system to perform the handoff with no interruption of communication. As an MS operates, it has to reregister periodically on the system by transmitting many parameters to the active BS while in an idle state. While a call is ongoing, the MS, BS and MSC cooperate to manage communications between BS and MS to maintain good radio link efficiency.
In CDMA (and wideband CDMA) technology, it is possible for a system to receive mobile transmission from more than two BS's at the same time. Further, an MS can receive transmissions from two or more BS's simultaneously. This feature makes it possible to efficiently handoff an MS from a BS to another BS or from a first sector within a cell to another sector within the cell using enhanced handoff techniques which maintain the quality of signal and voice information during handoff.
In a CDMA cellular and PCS system, many kinds of handoff are provided to guarantee the continuity of a call. The various handoff methods offer various tradeoffs related to efficiency and reliability of continuity of call and system load due to the method and implementation content. The establishment of a channel by a handoff operation is called add and the cancellation of a channel by handoff is called drop. Handoff methods are widely classified into soft handoffs and hard handoffs. A soft handoff operates in a make-before-cut method which guarantees continuity of call by establishing two or more simultaneous communication "highways" for one call. A hard handoff is a cut-before-make method which only uses one communication channel at a time. In a CDMA system, the majority of handoffs take the form of soft handoffs. In cases when a soft handoff cannot be achieved, the continuity of a call is guaranteed by a hard handoff.
In general, an MS determines when a handoff is required by evaluating a pilot-to-interference ratio (PIR) of a received pilot channel signal. When this value for any cell exceeds a minimum threshold value, this is reported by the MS and that cell is added to an "Active Set" of base stations. When the PIR of an active cell diminishes significantly relative to the other cells in the active set, that is reported by the MS and that cell is dropped from the active set. Typically, the add threshold is a fixed threshold value where the drop threshold is a relative threshold value based on the performance of all cells in the active set.
Because CDMA can establish multiple highways simultaneously on a given operating frequency by using different codes, soft handoff methods are particularly well suited for CDMA, which is capable of composing plural highways for one call. Because of the possibility of composition of two or more than highways simultaneously, the best call quality among multiple highways can be determined and then selected, whereby the whole quality and continuity of call is remarkably enhanced.
Soft handoffs are further divided into inter-cell handoff, inter-BSC handoff, and the like. Inter-cell handoff refers to simultaneously operating an ongoing call on two or more adjacent cells for one MS. Inter-sector handoff refers to a handoff between two sectors within the same cell. An inter-sector handoff is also referred to as softer handoff. Softer handoff is similar to soft handoff but there is a difference in source management and assignment. Because channel elements in a cell can handle highways of two sectors simultaneously, unlike soft handoff, softer handoff does not need the additional channel elements.
Soft handoffs occur when an MS resides in an area of overlapping cell coverage. The MS makes up traffic frames coming from each BS and transmits it into each BS again. Each BS then sends the received frame compound to the BSC, and the BSC applies frame quality metrics to select the best quality frame among the received traffic frames. This is referred to as selection diversity where the best of the N available copies of each frame are used. Frame selection is a very important algorithm because the quality of the selected frame can be a reason of cutting a call on an upper layer.
During a soft handoff, an MS is receiving signals from two or more BS. Each of these signals include reverse power control bits which instruct the MS to raise or lower its operating power in order to maintain a minimum effective power level. As the multiple sites are often not collected, the MS is likely to receive conflicting power adjustment instructions. To avoid undesirable signal deterioration in this condition, an MS only lowers its operating power when all cells in the active set report that the power of the MS should be decremented.
An additional consideration during a handoff is that the frames arriving from cells in the active set which are spatially separated do not arrive at the TSB simultaneously. As a result of the different physical path lengths between cells, the frames actually arrive with a slight delay between them. This condition is illustrated in FIG. 1. In the view of the time domain, even if there is only a slight time difference in the arrival time of the frames from the active set, this delay accumulates during the selection process rendering the total delay time excessive.
The step of selecting frames from the cells in the active set is performed by the TSB in the BSC. The selection process insures that if a status of one cell in the active set is bad while the status from another cell in the active set is good, reliable communication will be maintained by this function. The TSB carries out selection by comparing a sequence of frames currently being received with a sequence of frames previously received to determine the expected number of frames available for the selection process. During normal operation of a call, only one BS is handling the call for the MS, and the expected frame sequence is 1. During a soft handoff, several BS are in the active set, and the expected sequence is equal to the number of cells in the active set. However, the TSB is not aware of when cells are added or dropped from the active set and therefore must investigate when the sequence number changes prior to selection, thereby adding time to the selection process.
FIG. 1 illustrates an exemplary conventional frame selection operation viewed in the time domain. As illustrated, a frame A 101 arrives first, followed by a frame B 102 and then a frame C 103, each with a successive delay. The frames are selected by the TSB and then transmitted into a vocoder in the BSC only after the arrival of the last expected frame in the sequence, which in this case is frame C 103. The frame A 101, which arrived first is evaluated to determine whether selection is suitable or not. During this process, if a frame of another cell does not arrive within a predetermined time, the frame A 101 is considered unfit because only a single frame has been received. If the frame B 102 reaches the TSB after frame A 101 is considered unfit or during the period when the suitability of frame A 101 is being determined, the TSB repeats the selecting step. After frame C 103 arrives, the three frames are considered and finally the selection is performed.
The conventional TSB performs this selection process without any information about soft handoff. Therefore, the TSB has to investigate unknown frames of all cells and there are many losses due to this. For example, in the case where each frame arrives with a delay in a first cell and without a delay in another cell, it is possible that a sequence 2 from an undelayed cell arrives earlier than sequence 1 from a delayed cell. In this case, because more than one frame is received from the undelayed cell, satisfying the initial selection requirement of plural frames, the undelayed frames are selected without waiting for the frame of the delayed cell (sequence 1) nor giving consideration to the quality of the frames from the delayed cell. Once this occurs, frames from the delayed cell may be permanently excluded from selection even though that BS remains in the active set.
The problems just described associated with delayed arrival time are especially acute during a soft handoff between MSC's within a system. As the frames from various MSC's are trasnsported via a router, significant delay times are incurred
The selection initiation process operating as described above causes time loss, on account of checking up quality in each arrival of the frame and repeating the evaluation operation if no successive frame is received. Also, because the delayed frame is not used, eventually, the cell transmitting the delayed frame can be permanently excluded from selection of a frame.